Configure DiskGalaxy from runtime parameters

[mabruzzo]

This PR depends upon the changes in #523

---

This PR:

• consolidates DiskGalaxy so that a separate ClusteredDiskGalaxy type is no longer necessary (it has been removed)
• refactors DiskGalaxy so that it is now configured by runtime parameters.
• improves the way that we treat DiskGalaxy. It is now treated in a more normal way for models -- namely it is only constructed when relevant runtime parameters are provided (previously, it was always initialized)
• I took a lot of care to try to record the old values of the various paramters

New Parameters

The newly introduce ~24 new runtime configuration parameters. There are a few more that could be useful, but these are definitely the essentials.

These new parameters include:

• model.galaxy.cgm_init.profile_kind
• model.galaxy.cgm_init.T_anchor
• model.galaxy.cgm_init.rho_anchor_cgs OR model.galaxy.cgm_init.rho_anchor_Msun_per_kpc3
• model.galaxy.cgm_init.R_anchor_kpc
• model.galaxy.gas_disk.mass_Msun
• model.galaxy.gas_disk.scale_radius_kpc
• model.galaxy.gas_disk.selfgrav_scale_height_estimate_kpc
• model.galaxy.gas_disk.init.profile_kind
• model.galaxy.gas_disk.init.T_anchor
• model.galaxy.gas_disk.init.rho_anchor_cgs OR model.galaxy.gas_disk.init.rho_anchor_Msun_per_kpc3
• model.galaxy.star_forming_disk.global_sfr_Msun_per_kyr
• model.galaxy.star_forming_disk.poisson_point_process
• model.galaxy.star_forming_disk.kennicut_schmidt_power
• model.galaxy.star_forming_disk.earliest_t_formation
• model.galaxy.star_forming_disk.latest_t_formation
• model.galaxy.star_forming_disk.cluster_mass_dist.lo_Msun
• model.galaxy.star_forming_disk.cluster_mass_dist.hi_Msun
• model.galaxy.star_forming_disk.cluster_mass_dist.alpha
• model.galaxy.static_potential.halo.mass_Msun
• model.galaxy.static_potential.halo.concentration
• model.galaxy.static_potential.halo.virial_radius_kpc
• model.galaxy.static_potential.old_stellar_disk.mass_Msun
• model.galaxy.static_potential.old_stellar_disk.scale_radius_kpc
• model.galaxy.static_potential.old_stellar_disk.scale_height_kpc

Verbosity of parameters

At the moment parameter files look a have the structure

[model.galaxy.cgm_init]
# ...

[model.galaxy.gas_disk]
# ...

[model.galaxy.gas_disk.init]
# ...

But, if we adopt a real TOML parsing library (which is relatively easy if we merge #503), or I modify the parsing logic, we'll be able to write files of the form

[model.galaxy]
cgm_init.T_anchor= # ...
#...
gas_disk.T_anchor= #...
# ...

[hannahjleary]

Looks good besides the one typo in the docs!